For words that only have three letters, there is absolutely no 4th letter. As an example with the word “add”, there won't be any a lot more letters Later on and thus should be the word "incorporate". The word “addict” is within the BIP39 wordlist however, you would have utilised https://bookmarkahref.com/story18831181/new-step-by-step-map-for-bip39-english-words